Walk along the Willamette on a damp Portland evening and you might never guess what lives a few feet beneath your shoes. The city's stormwater grates, manholes, and aging clay sewer pipes form an enormous, climate-controlled apartment complex for one of the most successful mammals on the planet: Rattus norvegicus, the Norway rat. Portlanders have shared their city with this animal since the late 1800s, and today the relationship is, by most measures, getting closer.
The rat that isn't from Norway
Despite the name, the Norway rat is not Scandinavian. Wildlife biologists place its origin in the steppes of northern China and southern Siberia, from which it spread across the world aboard merchant ships. It arrived in the Pacific Northwest with the timber and trade boom of the late nineteenth century and never left. The Oregon Department of Fish and Wildlife notes that the species "nearly always resides near human activity," occupying houses, warehouses, sewers, and any other place that provides adequate shelter and a nearby source of food.
Norway rats are the chunky, ground-dwelling cousins of the more acrobatic roof rat. An adult is typically seven to ten inches long, not counting a scaly tail that's noticeably shorter than its body. Coarse brown fur shades to a dirty gray or yellow on the belly. They have small ears, blunt snouts, and a body built for digging, swimming, and squeezing through gaps no wider than a quarter. Their teeth register 5.5 on the Mohs hardness scale, which means they can gnaw through soft metals, lead pipe, and certainly the wood and plastic of an older Portland home.
A female Norway rat can begin breeding at two months old, produce litters of around eight pups, and have anywhere from three to twelve litters a year. Few live longer than a year in the wild, but they don't need to.
Reproduction is the heart of the problem. A small population in a well-fed sewer line can become a large one in a season, and Portland's sewer line is well fed.
A city built for rats
Portland's underground geography is, from a rat's point of view, close to ideal. The city's combined sewer system dates in part to the mid-nineteenth century, and although the celebrated Big Pipe project dramatically reduced overflows into the Willamette and the Columbia Slough by the early 2010s, miles of older clay and brick conduit remain in service. These older pipes crack, settle, and develop the small voids that rats love. Multnomah County Vector Control has been blunt about the relationship: rats are likely to be present wherever the sewer line is broken, and broken sewer line is the default condition in older cities.
Weather plays a role too. The Pacific Northwest's mild, wet winters mean rats rarely face the lethal cold snaps that thin populations in the Midwest, and heavy rains periodically flood the sewers and drive animals up and out in search of dry ground. Add a dense restaurant scene, abundant residential composting, and chicken coops in many backyards, and the food supply rarely runs short.
The result shows up in national rankings. Pest control giant Orkin has placed Portland on its list of the country's "rattiest" cities for several years running. The 2024 rankings put Portland in the top 20; the 2025 update, which crowned Los Angeles the worst rat city in America, again included Portland on its way up, climbing seven spots from the prior year. The trend is consistent enough that "Portland rat ranking" is now a category of journalism.
Inside the pipes
How many rats actually live in Portland's sewers? No one knows. "There's no way to even count how many rats are in Portland's sewers," Multnomah County Vector Control Specialist Chris Roberts told KATU in 2017. Roberts, at the time the county's lone "rat patrol" officer, said roughly ninety-five percent of complaints involved yards, basements, and attics rather than the sewer itself, but most experts agree the sewer is the reservoir from which surface populations are constantly replenished.
The sewers offer everything a rat needs: a steady ambient temperature, year-round water, darkness, and a buffet of food waste washed down through garbage disposals and street drains. From this base, rats fan out through cracks in lateral lines and storm drains, sometimes turning up in places homeowners would rather not find them. Portland-area pest controllers and the City of Portland itself acknowledge that sewer rats do, on rare occasion, climb up the plumbing into household toilets, using the small air pocket in the U-trap as a rest stop. Most never make it; the ones that do tend to make the local news.
The trap-shy rat problem.
A more troubling development is the rise of what some Portland exterminators call "EAD" rodents, for Equipment Avoidance and Disregard. Decades of trapping and poisoning, they argue, have selected for the most cautious, neophobic animals, which now decline to touch traps or bait. Pest and Pollinator, a Portland firm, has documented this behavior in downtown blocks with long histories of rodent control. The practical takeaway: in older Portland neighborhoods, a snap-trap-and-bait approach is increasingly likely to fail. Exclusion-first work — sealing the lateral, the cleanout, the vent — is doing more of the work than the trap is.
Quiet defenses
The city's response to its sewer rats is steady but unglamorous. The Bureau of Environmental Services contracts with Multnomah County Health Department's Vector Control program to monitor and treat rats and mosquitoes in the wastewater and stormwater system. A 2022 intergovernmental agreement budgeted roughly $3.42 million over its term to fund inspections, complaint response, and baiting at the bureau's facilities. Vector Control also offers free property inspections and covered snap traps to residents, and tracks broken sewer laterals — often the source of persistent yard infestations — by referring homeowners for dye tests.
Public health officials emphasize that the species is more than an aesthetic nuisance. Norway rats can carry leptospirosis, salmonellosis, murine typhus, and rat-bite fever, among other pathogens, and contaminate food and surfaces with urine and droppings. None of these are common in Portland on a per-capita basis, but the baseline risk is real, and the rat is the vector.
Still, eradication is not on the table, and never has been. The realistic goal in a city like Portland is suppression, performed quietly by county technicians, plumbers fixing cracked laterals, and homeowners installing rat flaps on their waste lines. Beneath the bridges, the Norway rat goes on with its busy, fertile, mostly invisible life — an old immigrant from the steppes of Asia that long ago decided Portland would do just fine.